Sleeptracker looks promising:

SleepTracker’s SmartStart Technology monitors your sleep stages throughout the night and then uses that data to determine the exact moment when you should be awoken helping you feel refreshed and energetic. SLEEPTRACKER® also comes with the most comprehensive sleep software available allowing you to see how you slept, ways to improve your sleep, and your overall sleep score.

However, at over $100 it feels a little bit pricy. I feel as if it'd be a great application on the Pebble App store in the future, and at a lower price point. The reviews on Amazon only help reinforce this point, highlighting that the watch works but the alarm just isn't loud enough to be effective in waking you up.